1. Field of the Invention
The invention relates to methods and apparatus for controlling a xe2x80x9clean burnxe2x80x9d internal combustion engine in a motor vehicle to achieve enhanced vehicle fuel economy while maintaining emissions compliance.
2. Background Art
The exhaust gas generated by a typical internal combustion engine, as may be found in a motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (xcex), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as HC, CO and NOx, based, for example, on detected values for instantaneous engine speed and engine load.
To limit the amount of feedgas constituents that are exhausted through the vehicle""s tailpipe to the atmosphere as xe2x80x9cemissions,xe2x80x9d motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NOx xe2x80x9ctrapxe2x80x9d. Both the upstream and downstream catalyst store NOx when the exhaust gases are xe2x80x9cleanxe2x80x9d of stoichiometry and release previously stored NOx for reduction to harmless gases when the exhaust gases are xe2x80x9crichxe2x80x9d of stoichiometry.
Because the trap""s actual capacity to store NOx is finite, the prior art teaches vehicle emissions compliance by discontinuing lean engine operation when the trap is deemed to have been xe2x80x9cfilledxe2x80x9d to a predetermined level, thereby avoiding the condition wherein feedgas NOx would pass through a xe2x80x9cfilledxe2x80x9d trap to effect an increase in tailpipe NOx emissions. Thus, in U.S. Pat. No. 5,473,887 and U.S. Pat. No. 5,437,153, a controller accumulates estimates of feedgas NOx, all of which is presumptively stored in the trap during a given lean excursion, and triggers a purge event when the accumulated feedgas NOx exceeds a predetermined threshold representing the trap""s nominal NOx-storage capacity. And because a trap""s instantaneous efficiency, i.e., the incremental rate at which the trap stores NOx present in the exhaust gas flowing through the trap, may begin to fall as the trap fills with NOx, U.S. Pat. No. 5,437,153 further teaches use of a nominal NOx-storage capacity which is significantly less than the actual NOx capacity of the trap, to thereby theoretically provide the trap with a near-perfect instantaneous NOx-absorbing efficiency as long as stored NOx remains below the nominal capacity.
Unfortunately, this prior art approach often discontinues lean engine operation while the trap still has meaningful available NOx-storage capacity, thereby degrading the fuel economy benefit to be achieved through lean-burn operation. Even further, while increasing the frequency of the purge events may serve to maintain relatively higher trap storage efficiencies, the fuel penalty associated with the purge event""s enriched air-fuel mixture and, particularly, the fuel penalty associated with an initial release of oxygen stored previously stored in both the three-way catalyst and the trap during lean engine operation, can further degrade the fuel economy benefits associated with lean engine operation.
It is an object of the invention to provide a method and apparatus for controlling the operation of a xe2x80x9clean-burnxe2x80x9d internal combustion engine which seeks to maximize the fuel economy benefit from lean engine operation while providing overall emissions compliance.
In accordance with the invention, a method and apparatus are provided for controlling the operation of an internal combustion engine in a motor vehicle, wherein the engine generates exhaust gas including NOx, and wherein exhaust gas is directed through an exhaust gas purification system including a NOx trap before being exhausted to the atmosphere. Under the invention, the method includes determining a distance effectively traveled by a vehicle during a first engine operating period, based upon the greater of an instantaneous vehicle speed and a minimum predetermined vehicle speed. Preferably, the minimum predetermined vehicle speed is characterized by a level of constituent emissions that is at least as great as the levels of constituent emissions generated by the engine when idling at a stoichiometric operating condition. In accordance with another feature of the invention, the resulting distance measure is reset upon commencement of a rich engine operating condition of a predetermined minimum duration.
In an exemplary method, the distance measure is determined using a value representative of instantaneous vehicle speed, either as directly detected or, for example, as estimated based upon engine speed and selected vehicle transmission (gear) ratio.
In accordance with another feature of the invention, the method preferably includes discontinuing a lean engine operating condition based on the first measure. More specifically, an exemplary method also includes determining a total amount of the exhaust gas constituent exhausted to the atmosphere during the first engine operating period; calculating an evaluative measure based on the total amount and the distance effectively traveled; and prohibiting a lean engine operating condition when the evaluative measure exceeds a predetermined threshold. By way of example only, in the exemplary method, determining the total amount includes periodically determining an instantaneous amount of the exhaust gas constituent being exhausted to the atmosphere, as through use of the output signal generated by a sensor positioned downstream of the device; and accumulating the instantaneous amounts during the first engine operating period. Preferably, the predetermined threshold is periodically adjusted based upon an indication of vehicle activity based, for example, on detected or determined values for vehicle or engine power.